1. Field of the Invention
This invention relates to a process and apparatus for chemical vapor deposition of diamond films.
2. Discussion of the Background
Deposition of diamond films using CVD techniques has been well established. Numerous workers have used a plethora of techniques and source gases for diamond growth (see T. R. Anthony in Mat. Res. Soc. Symp. Proc. 162, 61 (1990) or see also P. K. Bachmann et al. in Diamond and Related Materials 1,1 (1991)). The techniques have included microwave-plasma assisted, hot filament assisted, dc plasma assisted, arc-jet discharges, rf plasma assisted, and oxy-acetylene-torch CVD techniques. The vast majority of the work depends on molecular hydrogen dissociation/activation in high-temperature plasma regions or in equivalent high temperature regions such as a hot filament or an oxy-acetylene torch. As a consequence of the sample temperature being much lower than the source temperature, there exists a super equilibrium of atomic hydrogen at the diamond growth surface. Thus, diamond growth proceeds once a sufficient amount of atomic hydrogen is produced.
One role of the atomic hydrogen is to dissolve any graphite from the depositing diamond layer. Some of the earliest and simplest theories of diamond growth hypothesized that diamond CVD growth was a codeposition process involving the deposition of both graphite and diamond but in which the graphite was dissolved preferentially, resulting in stabilization of the diamond phase. Thus, providing an effective graphite etchant to dissolve graphite from a depositing diamond layer is critical in any diamond CVD process.
Another role of the atomic hydrogen is to promote diamond formation through stabilization of the surface of diamond. Additional insight into the thermodynamics of diamond deposition has been provided by W. A. Yarbrough whose quasi-equilibrium calculations have shown that, at high fractions of atomic hydrogen (greater than 0.1%), diamond condensation is preferred over graphite (see W. A. Yarbrough in Mat. Res. Soc. Symp. Proc. 192, 775 (1990)). Hence, diamond deposition techniques need to generate a high fraction of atomic hydrogen to insure diamond promotion over graphite. In addition, with the deposition process involving carbon atom addition per unit time, the deposition process must also provide a critical absolute atomic hydrogen flux per unit time in order to stabilize the instantaneous growth surface.
Many hydrocarbon, halo carbon, fluorocarbon, and organic sources have been used to produce diamond films. Typically, the promotion of diamond bonding over graphitic bonding is only accomplished when the percentage of hydrocarbon in the gas phase is small. A molecular hydrogen concentration insures high atomic H concentrations in the diamond growth systems. Thus, any graphite deposited by a particular technique can be dissolved before graphitic phases can be incorporated into the diamond. Correspondingly, the best films are deposited with hydrocarbon percentages between 0.5-2.0%. Films deposited at higher hydrocarbon concentrations show little if any evidence of diamond bonding from Raman analysis (see Hata and Sato in New Diamond, 32-34 (1990)).
For a number of reasons, workers have sought to alter, to modify, or to change the diamond process from a molecular-hydrogen based process. Workers have sought other sources of atomic hydrogen and have sought other graphite etchants (see T. R. Anthony, Mat. Res. Soc. Symp Proc. 162, 61 (1990)). Variations from the traditional 95-99% H.sub.2 with 1-5% CH.sub.4 feed gasses with F, Cl, O, and OH additives have been accomplished.
Of the variety of techniques for diamond growth which have been attempted for diamond growth, only the oxy-acetylene flames have had substantial success in the growth of high quality diamond films without the use of molecular hydrogen addition. Growth of diamond from oxy-acetylene flames has been accomplished using a 1:1.05 mix of O.sub.2 to C.sub.2 H.sub.2 as the premix entering the combustion flame (see L. M. Hansen et al. in Mater. Letters 7, 289 (1988)). In an oxy-acetylene flame, oxygen (O.sub.2) and acetylene (C.sub.2 H.sub.2) are spontaneously reacted in a chemical flame. Similar to the microwave plasma diamond growth, the chemical flame is at an extremely high temperature 3000.degree. C. At those temperatures, the reactants (O.sub.2 and C.sub.2 H.sub.2) and the burn products (CO, CO.sub.2, H.sub.2 O) are in a partially dissociated state such that atomic H is readily available to the diamond growth surface. The oxy-acetylene flames which produce diamond burn "rich". The position of substrate in the flame has always been inside the primary flame where H.sub.2 and CO are present. Diamond growth in the secondary flame containing H.sub.2 O and CO.sub.2 has not been observed. The secondary flame is an oxidizing atmosphere. Y. Hirose (Applications of Diamond Films and Related Materials, Materials Science Monograph 73, 471 (1991)) has pointed out that "the combustion flame comprises two areas, one being an oxidizing area called the `outer flame` (oxidizing one) and the other being a reducing area called `inner flame` (reducing one)." Furthermore, Hirose states that "it is a well known experimental fact that a key point to successful diamond synthesis is to produce a radical which is made in a reducing plasma."
O and OH chemistries have been also exploited in both plasma-assisted and hot-filament techniques. For example, small quantities of oxygen and water vapor have been added to microwave plasma reactors for the purpose of oxygen addition. Small percentages (0.5-2%) of oxygen and small percentages of water vapor (0-6%) improve the Raman spectra and decrease the temperature at which diamond deposits (see Saito et al. in J. Mat. Sci. 23, 842 (1988) or Saito in J. Mat. Sci. 25, 1246 (1990)). However, higher percentages have been observed to degrade the diamond quality. Likewise, Aoyama et al (Diamond and Related Materials 2, 337 (1993)) reports that, for plasma jet systems, "Small amounts of oxygen contained in the plasma jet promote the purification of deposited diamond. However, larger amounts of oxygen atoms or molecules react with diamond, and the deposition of diamond is suppressed."
Workers experimenting with hot-filament apparatus have reached the same basic conclusions. Kawato and Kondo (Jap. J. Appl. Phys 26, 1429 (1987)) have shown that the deposition of non-diamond phases is suppressed so that the quality of diamond is improved when small amounts of oxygen are added (less than 4%). However, Matsumoto (Proceedings of the Electrochemical Society 89-12, 50 (1989)) cautions not to use a "heated filament in an atmosphere of high oxygen content." Furthermore, Matsumoto states that "oxygen concentrations higher than 33% burn off the hot filament."
Indeed, high percentages of oxygen as O.sub.2, OH, or O are deleterious to many CVD diamond growth systems. Systems which use hot-filaments or hot metal fixtures (such as arc discharges) are particularly susceptible to metallic erosion. Materials such as W, Mo, Tn, Rh form volatile metallic oxides which sublime in the vacuum system. This sublimation limits the lifetime of the apparatus as well as contributing to metal contamination in the diamond film. Indeed, H. Chen et al. reported that the lifetime of a W filament is reduced when oxygen concentrations exceed 1% (H. Chen et al. Applications of Diamond Films and Related Materials, Materials Science Monograph 73, 137 (1991)). However, Komaki and Hirose (Jap. Pat. Kokai Sho 62 [1987]-180060) have reported the growth of diamond-like carbon and/or diamond using high concentrations of water across a heated W filament.
In general, oxygen is a very reactive gas with a broad spectrum of materials. Once the temperature of the material exceeds the oxidation temperature of the material, the material is either converted to a stable oxide, or the material sublimes. Two examples are given. First, elemental tungsten is an extremely robust high temperature material in vacuum or in an inert atmosphere. Tungsten melts at 2900.degree. C. However, in an oxidizing atmosphere (such as an environment containing significant O.sub.2, or H.sub.2 O), tungsten forms a volatile oxide which sublimes at 800.degree. C. and melts at 1500.degree. C. Second, carbon is also an extreme high temperature material which melts at temperatures greater than 3000.degree. C. Carbon is rapidly oxidized at 800.degree. C. Diamond, graphite, coals, and other forms of carbon all react with H.sub.2 O and O.sub.2 to form CO at temperatures of 700.degree.-900.degree. C.
Hence, the distinct problem associated with the growth of diamond from feed stock containing a high oxygen content is the amount of reactive oxygen (O.sub.2, O, H.sub.2 O, or OH) existing at the diamond growth surface.